fix: add SPA rewrite rule for Vercel deployment

Without this, Vercel serves a 404 for any client-side route accessed
directly (e.g. /admin/login, /shop) since it has no server-side route
matching those paths — only index.html exists as a static file. React
Router then never gets a chance to handle the path client-side.

Claude-Session: https://claude.ai/code/session_01NhxA4npAXAxmCDEJt6kxSH

vercel.json

@@ -0,0 +1,5 @@
+{
+  "rewrites": [
+    { "source": "/(.*)", "destination": "/index.html" }
+  ]
+}